1. Field of the Invention
The present invention relates to particulate materials with a protective water vapor-barrier coating. More specifically, the invention relates to coated particulate fertilizers such as ammonium nitrate with a blend of amines and microcrystalline and/or synthetic waxes.
2. Discussion of the Background
Particulate fertilizers, stored in bulk, will absorb moisture from the atmosphere at rates depending on the composition of the chemical salt, temperature of the solid and the vapor pressure of water in air (Physical Properties of Fertilizers and Methods for Measuring Them, Bulletin Y-147, National Fertilizer Development Center, October 1979). It is generally understood that the quality of particulate fertilizers can be improved by reducing their moisture content during production and by preventing moisture absorption during storage. This is especially true for fertilizers stored in large bulk piles where no dehumidification is provided.
Various coatings have been proposed for fertilizers. Obrestad et al, U.S. Pat. No. 5,472,530, disclose coatings designed to produce free flowing ammonium nitrate particles. Simms et al, U.S. Pat. No. 4,521,239, disclose protective coatings for ammonium nitrate comprising a combination of an amine and a polysiloxane. Other coatings for fertilizers are disclosed by Novascues et al, EP 00574306; Kjohl et al EP 00255665; and Ureng et al U.S. Pat. No. 5,294,251. These coatings are taught to provide limited uptake of water by the fertilizer stored in bulk in the open air in a wet atmosphere, limit the formation of significant encrustation, reduced dust formation and hygroscopicity, corrosion resistance properties and water proofing properties.
Most commercially available coating technologies for fertilizers such as ammonium nitrate claim to provide anti-caking properties and even moisture barrier properties. However, the mechanism for their efficacy is generally not discussed other than saying that the materials are hydrophobic. Common commercially available coating agents include Galoryl alkyl-aryl sulphonates, anionic and nonionic surfactants (sold by Lobeco Industries), Lilamine and Armoflo fatty amine coatings (sold by Akzo Nobel), Petro-Ag (an amine based coating marketed by Witco Corporation), Perflow (another amine based coating marketed by IWC Stockhausen) and Radiamine (sold by Fina Chemicals).
Solid fertilizers, and particularly ammonium nitrate, are usually coated with active anti-caking agents. These anti-caking agents serve to reduce the contact area between particles where fertilizer salts are soluble and where these salts may recrystallize from saturated aqueous film layers between particles (D. C. Thompson, 1972). However, commercially available anti-caking agents tend to be poor water vapor barriers.
In the case of ammonium nitrate, the anti-caking agent may serve to prevent significant caking in storage, but moisture will continue to be absorbed through the coating layer, causing saturation of internal additives and desiccant phases. Once these desiccant phases become saturated, undesirable crystal phase transitions can occur during thermal cycling resulting in product degradation. This is evidenced by fluff formation over the surface of bulk piles, crust layer formation beneath the fluff layer and deep pile caking as moisture migrates vertically in the bulk pile.
Accordingly, one object of this invention is to provide a suitable anti-caking agent that also has superior water vapor barrier properties.
Another object of the present invention is to provide a coated particulate fertilizer that is protected from caking and from moisture absorption by the coating.
This object and other objects of the present invention have been satisfied by the discovery of a process for application of amines or blends of amines and at least one of microcrystalline waxes, paraffins and/or synthetic waxes to particulate fertilizers which provide excellent resistance to caking in bag and bulk storage and the resulting coated particles provided. Furthermore, these blends provide resistance to water vapor absorption by the fertilizer thus increasing the storage life of the product.
Any material which is to be successful as a coating agent for particulate fertilizers must possess several desirable characteristics. The coating agent must be compatible with the fertilizer material, i.e., it must not cause deterioration of the fertilizer nor render it hazardous, which is a particular concern with ammonium nitrate. Since the coated fertilizer is frequently handled by unprotected personnel and is applied to crop land, the coating agent must be nontoxic. In some instances where a hygroscopic salt is being coated such as in the case of ammonium nitrate, the fertilizer readily absorbs water from the ambient air. This results in caking and breakdown of the prilled fertilizer. Accordingly, an acceptable coating agent must substantially impede or eliminate moisture absorption by the particulate hygroscopic fertilizer. Another important aspect of the coating is that the coating on the particles must be sufficiently uniform to prevent particle-to-particle contact in uncoated areas of the particles. If this type of contact is not prevented during storage of the ammonium nitrate, caking of the fertilizer particles will result as bonding occurs between particles where there is insufficient coating material to prevent exposed surfaces of the particles from contacting each other. The raw materials for the coating agent must be in abundant supply so that production of coated fertilizer product in volume is not impeded. The coating agent must firmly bond to the fertilizer particles from the time it is applied until the coated fertilizer is applied to the crop land.
Other beneficial characteristics which the coating agent should possess, but which are not mandatory, are that the cost of the ingredients on a per ton fertilizer basis should be reasonable and it should be able to be easily handled from an operational standpoint. However, with regard to both of these factors, some expense and inconvenience can be tolerated as long as a superior coated fertilizer product is produced.
The coating agent of the present invention has been found to satisfy all of the above-described characteristics. The present coating agent is a mixture of an amine and a microcrystalline and/or synthetic wax.
The amine component of the present coating composition is a material which is semisolid or solid at room temperature. The amine should melt at a temperature of about 43xc2x0 C. to about 93xc2x0 C. and should not decompose at these temperatures. Suitable amines include primary, secondary and tertiary, straight and branched chain amines of 12 to 18 carbon atoms per carbon chain, as well as amines containing cyclic hydrocarbon radicals of 5 to 8 carbons. Saturated cyclic amines and saturated polyamines may also be used as long as they possess the above-desired physical properties. Mixtures of amine may also be used, and in fact, long chain fatty amines as obtained industrially are frequently a mixture of amine compounds and are quite suitable for use in the present invention. Amines containing unsaturated hydrocarbon and aromatic hydrocarbon radicals should be avoided when coating the likes of ammonium nitrate because of the increased danger of forming hazardous compositions. This is substantially much less of a factor when relatively less hazardous substrates are to be coated. A preferred amine composition is a mixture of amines having the formula:
CH3xe2x80x94(CH2xe2x80x94CH2)xxe2x80x94CH2xe2x80x94NH2 
wherein x is 6, 7 and 8. This particular mixture is characterized as follows:
(a) Solid at 24xc2x0 C.
(b) Melting point (xc2x0 C.): 49xc2x0-54xc2x0 C.
(c) Freezing point (xc2x0 C): 43xc2x0 C.
(d) Specific gravity (150/60), g/cc: 0.794
(e) Viscosity (66xc2x0 C.) cp:3
(f) Colorless
(g) 97% purity
The wax component of the coating composition should be a soft or hard microcrystalline wax, a paraffin wax or a soft synthetic wax. The wax component must be either miscible with the amine component to provide a single coating composition or be able to be coated simultaneously or sequentially along with the amine component onto the particulate fertilizer as part of a two part coating composition. Microcrystalline waxes are the product of tank bottoms from crude oils that have been dehydrated, deoiled, and depending on grade, decolored. In comparison to paraffin wax, microcrystalline waxes have higher melting points, a lower percentage of normal alkanes, more iso- and cyclo-alkanes, higher molecular weights, higher flash points and broader melting point endotherms. The hardness of microcrystalline wax can be altered by a number of factors including oil content and the quantity of other additives such as polyethylene. These and other properties of microcrystalline waxes are described in Tuttle, The Petroleum Waxes in Petroleum Products Handbook and Unmuth, Petroleum Waxesxe2x80x94Their Composition and Physical Properties, CSMA Convention, Chicago, May 1975. The entire contents of these references are incorporated herein by reference.
Synthetic waxes are paraffins that are produced by the Fischer-Tropsch reaction where coal is burned in gas generators in the presence of oxygen to yield CO and H2. The gas stream is then converted to hydrocarbons over an iron catalyst and the resulting products refined to various synthetic-paraffin grades of wax. The synthetics have very narrow ranges of physical properties. Petroleum waxes generally have hydrocarbons in the C18-C70 range. Microcrystalline waxes, on the other hand, have hydrocarbons in the C36-C70 range with small to very small needle-like crystals. Paraffin waxes have hydrocarbons in the C18-C56 range with plate-like crystals.
Preferred microcrystalline waxes should melt at a temperature of about 60xc2x0 C. to about 100xc2x0 C., have a ATSM needle penetration value of about 3 to 60 at 25xc2x0 C. and are composed of 80 to 95% normal alkanes and 20 to 5% non-normal alkanes. Mixtures of microcrystalline waxes may also be used. More preferred microcrystalline waxes melt at a temperature of 74 to 96xc2x0 C., a ATSM needle penetration of about 3 to 35 at 25xc2x0 C. and are composed of 85 to 93% normal alkanes and 15 to 7% non-normal alkanes. Commercially available microcrystalline waxes that meet these criteria are Be Square 195 Amber, FR 5315, Victory Lite microcrystalline wax, C1035 microcrystalline wax, and SP200 soft synthetic wax, all available from Bareco Products of Rock Hill, S.C.
The amine and wax coating ingredients may be applied to fertilizer particles by any method which is industrially acceptable for providing the particles with a uniform and acceptable coating of the coating agent on the fertilizer particles. Both batch and continuous procedures can be used to mix the ingredients of the present composition. Since the amine used is most commonly solid at ambient temperatures, it is usually heated to a temperature sufficient to liquify the same. A temperature ranging from 54xc2x0 to 93xc2x0 C. normally is sufficient to liquify the amine. The amine is then mixed with the wax in the amount desired for a time sufficient to obtain a homogeneous mixture. Mixing may be done in a simple stirred tank, in an in-line static mixer or other such similar devices sufficient to prepare a homogeneous mixture of the two ingredients. The mixing device as well as all production apparatuses should be made of materials which are not corroded by the reactants. The amount of mixed material prepared should be sufficient to coat a given quantity of fertilizer particles to a given depth of coating agent.
The amount of amine combined with the wax can vary over a range with the essential requirement being a ratio of reactants which yields a coating composition having the characteristics stated above. A broad ratio range of the two reactants can be used with an acceptable amine:wax ratio ranging from 5/95 to 95/5. Ratios on either side of these limits can be used, however there is no advantage to be gained by exceeding these limits. A preferred operating range is an amine:wax ratio range of 70/30 to 90/10.
The coating mixture prepared may be used immediately or it may be stored for later use. The nature of the coating mixture is such that personnel when handling the product should use suitable protective clothing. Preferably, the coating mixture is stored for no longer than a few days in the molten form since at elevated temperatures (above ambient) oxidation or decomposition is promoted. If the coating mixture is to be stored for a period of time longer than several days, the mixture should be solidified by cooling, and then remelted before use. Storage under an inert atmosphere of a gas such as nitrogen is preferred.
Neither the coating mixture of the invention nor the amine reactant should be allowed to come into contact with hot ammonium nitrate melt. Because the amine and the wax are both organic materials, contact of the amine or the wax with hot ammonium nitrate could produce a potentially hazardous situation. For this reason, all spills and residual ammonium nitrate with applied coating should be adequately disposed of and not recycled to the ammonium nitrate plant. For fertilizer materials which are not under such hazardous constraints, these precautions can be reduced or eliminated.
The coating mixture once prepared and ready to use is in the liquid state. The mixture can be applied to the particulate fertilizer by any acceptable and reasonable method of applying a liquid to a particulate solid substrate in high volume commercial amounts. Usually, the warmed mixture is applied by such techniques as spraying, film rolling, spraying the liquid mixture onto a rotary drum onto which the fertilizer particles are dropped, and the like. The coating mixture must be kept at a temperature high enough so that the coating mixture remains liquid during application to the fertilizer particles. A temperature within the range of about 54xc2x0 to about 93xc2x0 C., preferably 65xc2x0 C. to 82xc2x0 C. is sufficient to keep the mixture molten. Unlike some coating processes, it is not necessary to heat the fertilizer particles. In the case of ammonium nitrate fertilizer, the particulate product as obtained from production is at an elevated temperature. From the production standpoint, the particulate ammonium nitrate product can be conveniently coated while at a temperature ranging from 31xc2x0 C. to 35xc2x0 C. The nature of the coating mixture is such that it can be easily applied to ammonium nitrate fertilizer having a temperature range of 16xc2x0 to 110xc2x0 C.
No matter what method is used to coat the fertilizer particles, the only relatively critical factor with respect to contact time of the liquidified coating agent with the particles is that contact should only be long enough to obtain a uniform coating. When the roll coating technique is used, the particles should only roll a distance sufficient to obtain a uniform particle coating.
Once the particles are coated, they only need to be cooled before being bagged. In the event the coating is applied to cool fertilizer particles, no further cooling of the particles is necessary. In the case of ammonium nitrate forced convective cooling within the coating apparatus may be utilized, but is not necessary. Cooling of the particles, however, aids in the solidification of the applied coating agent. The coating agent will solidify by the conduction of heat away from the coating by the relatively cooler underlying fertilizer substrate when the fertilizer particles have been cooled to within the preferred temperature range. If convective cooling of the coated particles is employed, care must be taken to not cool the particles so fast that the coating agent does not have sufficient time to uniformly coat the particles.
The coating conditions are such that the weight of coating applied is that which provides a sufficiently protective coating. Usually, an amount ranging from 0.005 to 0.20% by weight is sufficient to give good results, with a more preferred amount ranging from 0.01-0.05% by weight. If the coating is less than 0.005% by weight, the particles may not be coated with a sufficient amount of coating agent to provide a uniform coating having the necessary characteristics described above. If the coating agent is applied in an amount in excess of 0.20%, additional costs are incurred without any further benefits being realized.
The coating agent of the invention may be utilized to coat materials other than fertilizer. Indeed, just about any particulate solid material which has a tendency to cake may be coated. The solid being coated must be compatible with the coating agent.
Having generally described this invention, a further understanding can be obtained by reference to certain specific examples which are provided herein for purposes of illustration only, and are not intended to be limiting unless otherwise specified.